1. Field of the Invention
The present invention relates generally to a pointing unit included in a pointing device such as a stylus pen associated with a coordinate data input device such as a digitizer or tablet which has been commonly used as one of many input devices for a computer system. Especially, the present invention relates to improvement in the pressure sensitive mechanism of a stylus pen and a stylus pen including the pressure sensitive mechanism.
2. Description of the Prior Art
Conventional stylus pens (hereinafter, referred to as "pen") used as a pointing device for a digitizer have been constituted for detecting pen-down information in additon to coordinate information on the pointing device. Such pen-down information represents one specific condition of a pen point of the stylus pen being in contact with a panel of the digitizer. The pen-down information is detected by pressure sensitive means for detecting the vertical pressure applied to the pen point.
As a typical point detecting system for a digitizer, an electromagnetic signal utilizing system has been commonly used. This system comprises a sensor panel including a plurality of sensor coils arranged parallel to the detecting direction, and a pointing device such as a pen or a cursor. Since the pointed place is represented by X and Y coordinates, the detecting area of the sensor panel is composed of two sections intersected perpendicularly to X and Y directions. The pointing device is provided with a coil or resonant circuit to perform electromagnetic interaction between the pointing device and the sensor coils. A typical detecting operation of such constitution will be explained. These sensor coils in the sensor panel transmit an electromagnetic wave and then receive a response electromagnetic wave re-emitted by the interaction between the transmitted electromagnetic wave and the resonant circuit in the pointing device. The coordinate values of the pointing device are produced by arithmetic operation according to a main sensor signal generating the strongest electromagnetic energy and supplemental sensor signals adjacent to the main sensor.
Such a constituted electromagnetic signal utilizing system provides some advantageous features, one of which allows the pointing device to be used without a connection lead; i.e., cordless and free from a power source such as a battery. The applicant of the present invention has already provided various digitizers according to this electromagnetic signal utilizing system as shown in Japanese Patent Publication No. 2-53805/1990 and Japanese Patent Application Open-Publication No. 3-147012/1991.
Further various modified detecting systems are known. For example, a sensor panel transmits an electromagnetic wave and a pointing device receives this electromagnetic wave. Reversely, a pointing device transmits an electromagnetic wave and a sensor plate receives the transmitted signal.
In these systems utilizing electromagnetic mutual function, a pointing device includes a coil or resonant circuit. Furthermore, a stylus pen type pointing device may be provided with a pen pressure sensing function to sense pen pressure, and/or on-off switching mechanism to input switching information at a required position. In such a type of stylus pen, the pressure sensing function and the switching mechanism are performed by changing the resonant condition from normal position of the coil or resonant circuit.
Therefore, various types of stylus pens having such pen pressure sensing function have been proposed. For example, such a pen pressure sensing mechanism is provided by replacing a part of the capacitor or coil of the above described resonant circuit with a variable capacitor or variable coil, or adding a variable resistor to the resonant circuit. These variable elements are easily varied in response to the pen pressure. Such a pen pressure sensing type stylus pen is disclosed in Japanese Patent Application Open-Publications No. 63-257823/1988, No. 64-53223/1989, No. 4-96212/1992, and No. 5-275283/1993.
FIG. 1A to FIG. 1C show conventional stylus pens, known as L (inductance) variable type stylus pen, including the resonant circuit for detecting pen pressure.
These three stylus pens include the same components such as an external casing 12, as a pen holder, formed in a cylindrical shape, a core member 14 arranged on the axis of the external casing 12, a core support member 16 composed of a ferrite core with a through hole in which the core member 14 is slidably supported, a coil 20 wound around the circumferential surface of the ferrite core 16, a movable magnetic member 18 capable of moving with respect to the ferrite core, an elastic member or coil spring 22, and a capacitors 24. The core member 14 is formed in a substantially cylindrical shape with a tapered end, as a pen point, to facilitate pointing operation by contact between the tapered end and a sensor plate. Although these stylus pens may be provided with additional members such as a switching unit and the like, such additional members are not shown. In these stylus pens, the ferrite core 16 is fixed to the external casing 12 so that the core member 14 can be moved upwards in the axis of the external casing 12 when the pen point is pressed on a sensor plate (this operation is referred to "pen down"). In these stylus pens, a movable magnetic member 18 is associated with the core member 14 so that the movable magnetic member 18 can be integrally moved with the movement of the core member 14. In detail, the stylus pen shown in FIG. 1A includes a movable magnetic member 18 fixed to the side surface of the core member 14, thereby allowing integral movement. The stylus pen shown in FIG. 1B includes a movable magnetic member 18 fixed at the rear end of the core member 14. The stylus pen shown in FIG. 1C includes a movable magnetic member 18 surroundingly assembled on the front section of the ferrite core 16.
In such constituted stylus pens, as the movable magnetic member 18 is moved by the pen down operation, the distance defined between the movable magnetic member 18 and the ferrite core 16 will be also varied. In FIG. 1B and FIG. 1C, "di" represents an initial distance between the movable magnetic member 18 and the ferrite core 16 when the stylus pen is free from pen down pressure. On the other hand, the coil 20 and the capacitor 24 constitute a resonant circuit. The resonance conditions for this resonant circuit are determined so as to make resonance with the electromagnetic wave transmitted to and received from sensor coils. In other words, inductance of the coil 20 depends on the distance between the ferrite core 16 and the movable magnetic member 18. As a result, any pen down operation can be detected by watching change in the resonance conditions.
The elastic member 22 is not only limited to a coil spring, but an elastic rubber may be also used.
Furthermore, another modified stylus pen provided with an eraser function in addition to the above described functions is known. Still another modified stylus pen equipped with two different pen points having different functions at both ends has been provided to aim at increasing the information capacity to be input through the stylus pen. However, these modified stylus pens must reduce the dimension of the pen pressure sensitive mechanism because the whole length of the stylus pen is limited to a specific length, for example 140 mm, owing to the view point of the design. In the stylus pen provided with a pen unit at one end and an eraser unit at the other end, it is desirable to set the center of gravity at the pen unit's side to facilitate operator working. Therefore, it is preferable to reduce the weight and length of the eraser unit or the second pen point as much as possible.
The above described conventional stylus pens having the pen pressure sensitive structures shown in FIG. 1A to FIG. 1C can not be sufficiently minimized because the coil spring of the pen pressure sensitive unit requires a relatively long stroke space. Even if the coil spring is replaced by an elastic rubber, greater space will be required to provide a light pen pressure characteristic. Furthermore, since the hardness of the rubber member is required to be smaller to provide a light pen pressure characteristic, the rubber member will be deformed greatly owing to the compressive permanent set of the rubber member itself.